Epidural oxygen sensor

ABSTRACT

A sensor for measuring the oxygen saturation of blood flow within the skull is described. In a first embodiment the sensor comprises a photodetector and a pair of light emitting diodes surface mounted near the end of a length of flexible printed wiring. The sensor is hermetically sealed by a coating of rubber or polymeric material which has an optical window over the photodetector and light emitting diodes. The sensor is inserted through a burr hole drilled in the skull and slides between the skull and the dura of the brain. The light emitting diodes are pulsed to illuminate blood within the dura and brain with light, and light reflected by the blood is received by the photodetector and converted to electrical signals. The signals are processed by a pulse oximeter to provide an indication of blood saturation. In a second embodiment the photodetector and light emitting diodes are mounted at the end of a core of compressible foam extending from the end of a hollow bone screw. As the bone screw is screwed into a burr hole in the skull the photodetector and light emitting diodes will contact the dura and the foam will compress to maintain optical contact between the electrical components and the dura. Light from the diodes is reflected by blood in the dura and brain, received by the photodetector, and the resultant electrical signals are processed by the pulse oximeter.

This is a division of application Ser. No. 394,997, filed Aug. 17, 1989,now U.S. Pat. No. 5,024,2246.

This invention relates to sensors for determining the oxygen saturationof tissues within the skull and, in particular, to such sensor which areplaced epidurally through the skull to measure oxygen saturation.

During neurological and neurologically related surgical procedures it isoftentimes desirable to continuously monitor the oxygenation of bloodwhich is supplied to the brain. Frequently access is gained to the brainthrough a borehole in the skull, and a sensor which measures oxygenationcan then be inserted through such a borehole. A sensor should thenexhibit numerous design and performace criteria in order to operatesatisfactorily in this environment. The sensor must be capable oofinsertion through the borehole so as to contact tissue where oxygensaturation is to be measured. The sensor must be soft so that it doesnot damage neurological tissue, yet be sufficiently rigid in certaindimensions so that it can be maneuvered from outside the skull. It alsomust be sized to fit inside the borehole and in the location wheremeasurements are to be taken. Furthermore, the sensor must be designedso as to eliminate detection of ambient light which will interfere withdetection of the desired optical signals. The sensor must also preventthe detection of directly transmitted light from the light source of thesensor.

In accordance with the principles of the present invention, an opticalsensor is provided for epidural measurement of blood oxygenation. In afirst embodiment the sensor comprises a pair of light emitting diodes(LED's) which emit light at two predetermined wavelengths. The sensoralso includes a photodetector for receiving light emitted by the LED'swhich has been reflected from adjacent blood perfused tissue. The LED'sand the photodetector are mounted on flexible printed wiring whichtransmits signals to the LED's and from the photodiode. The componentsare encapsulated in a soft polymer which is biocompatible. The resultantsensor is thus capable of operation in an epidural environment, and isfurther capable of being measured into the desired position for epiduralmeasurements.

In a second embodiment the LED's and photodetector are located in ahollow bone screw, with the components opposing the tissue from whichmeasurements are to be taken. The components are backed by a softpolymer which will compress under gentle pressure as the bone screw istightened to cause the components to contact the dura.

In the drawings:

FIG. 1 illustrates a cross-sectional view of the use of an epiduraloxygenation sensor constructed in accordance with the present invention;

FIG. 2 is a side cross-sectional view of an epidural oxygenation sensorconstructed in accordance with the principles of the present invention;

FIG. 2a is a perspective view of an epidural oxygenation sensorconstructed in accordance with the principles of the present invention;

FIGS. 3a-3c are cross-sectional views of different embodiments ofepidural oxygenation sensors of the present invention;

FIGS. 4a-6c are plan views of different placements of LED's andphotodiodes of epidural oxygenation sensors of the present invention;

FIG. 7 is an electrical schematic of the components of the epiduraloxygenation sensor of FIG. 2; and

FIGS. 8a-8c are cross-sectional, top, and bottom views of an epiduraloxygenation sensor mounted in a hollow bone screw.

Referring first to FIG. 1, a skull is shown in which a burr hole 12 hasbeen drilled. Underlying the skull is the dura 16 which encases thebrain, and beneath the dura is the cerebrum 14. An epidural oxygenationsensor 20 is inserted through the burr hole 12 for measurement of theoxygenation of blood flowing in the dura 16. The sensor 20 is insertedthrough the burr hole and slides between the skull 10 and the dura 16,where is is shielded from ambient light entering the burr hole. At thedistal end of the sensor 20 is a photodetector 24 and LED's 22 whichface the dura through optical windows in the sensor. The photodetectorand LED's are mounted on flexible printed wiring which is connected to asensor cable 26. The sensor cable is connected to a pulse oximeter (notshown), which provides drive pulses for the LED's, receives electricalsignals fro the photodetector, and processes the received electricalsignals to produce an indication of the oxygen saturation of blood inthe dura. The sensor is operated in a reflective mode, whereby light atdifferent wavelengths emitted by the LED's is reflected by the blood inthe dura and the reflected light is received by the photodetector.

As shown in FIG. 2, the sensor 20 comprises a photodetector 24 and anadjacent pair of LED's 22a and 22b which are surface mounted to leads offlexible printed wiring 28 such as 0.001 inch Kapton™ wiring. The use ofsurface mounted components and the printed wiring provide a thin sensorwhich minimizes cerebral compression. Separating the LED's and thephotodetector is a light barrier 25 which prevents the directtransmission of light from the LED's to the photodetector. The lightbarrier may be provided by an opaque epoxy material, but in a preferredembodiment the light barrier is formed of a thin sheet of copper foil.The copper foil not only effectively blocks light from the LED's, but isalso connected to a grounded lead of flexiblle printed wiring. Thecopper foil thus shields the photodetector from radio frequencyinterference such as that emanated during pulsing of the LED's.

The foregoing components are encapsulated by a soft coating 30 ofsilicone rubber or polyurethane material. The soft coating smoothlyrounds the corners and edges of the sensor which prevents injury to thedura by the sensor. The coating also hermetically seals the componentsfrom moisture and other environmental factors. The coating 30isoptically transmissive to light at the wavelengths of the LED's where itoverlies the lower surfaces of the photodetector and the LED's fromwhich light is transmitted and received by these components.

FIG. 2a is a perspective view of the sensor 20 of FIG. 2, referenced tox, y, and z axes. As mentioned above, the coating 30 provides the sensorwith a smooth, gently rounded profile such as the rounded distal end 27.The sensor is relatively stiff along the portion of the printed wiringwhere the components are mounted to maintain their relative alignment.In the x dimension the sensor is fairly stiff so that it may be insertedand guided beneath the skull and in contact with the dura. In the zdimension the sensor is stiff to provide maneuverability duringplacement of the sensor. In the y dimension the sensor proximal thecomponents is flexible to curve through the burr hole and under theskull, which may have a thickness of 2 to 20 mm depending upon thepatient.

In order to be capable of sliding between the skull and the dura thesensor should be thin in the y dimension so as not to injure thepatient. Preferably the sensor thickness in this dimension should not begreater than 4 mm, and most preferably not greater than 2.5 mm. Thesensor should also not be less thane one millimeter in thickness tomaintain continuous contact with the dura. This will reduce theoccurrence of motion artifacts, as the dura can move as much as 1/2 mmaway from the skull during hyperventilation of the patient, forinstance.

In the embodiment of FIG. 2 the photodetector 24 is located toward thedistal end 27 of the sensor with respect to the LED's 22. This distalplacement of the photodetector keeps the photodetector well removed fromthe burr hole and ambient light passing through the burr hole. FIGS.3a-3c show other component orientations which may beemployed indifferent sensor embodiments. In FIG. 3a the LED's 22' are canted towardthe dura where the dura overlies the photodetector 24, which improvesthe efficiency of light reflectance. The canted LED's are supported by afiller of the coating material 30. In FIG. 3b two pairs of LED's 22" arelocated on either side of the phototdetector 24 to illuminate the durafrom both sides of the photodetector. In FIG. 3c the pair of LED's 22 iscentrally located between a pair of photodetectors 24', the latter beingcanted toward the area of the dura illuminated above the LED's.

It is desirable for the optical windows of the components which face thedura to be as large as possible so as to maximize optical transmissionefficiency. Opposing this desire is the constraint that the sensor mustbe sized to fit through the burr hole in the skull. To determine thelargest components which may fit through a given burr hole dimaeter,rectangular configurations of components may be calculated which arecapable of fitting through the burr hole. FIGS. 4a-6c show componentconfigurations which can fit through a 14 mm diameter burr hole. FIGS.4a-4c show plan views of component layouts for the single photodetectorand pair of LED's employed in the sensors oof FIGS. 2 and 3a. In FIG. 4athe LED's 22' and the photodetector are arranged in a layout whichmeasures 8.2 mm by 6.3 mm. In FIG. 4b the rectangular layout measures10.5 mm by 5.3 mm, and in FIG. 4c the rectangular layout measures 9.3 mmby 5.7 mm. In each layout the coating material 30 is shown in the areaoutside the boundaries of the electrical components.

FIGS. 5a-5c show component layouts for a 14 mm diameter burr hole usingtwo pairs of LED's 22" and one photodetector 24. In FIG. 5a therectangular layout measures 10.8 mm byy 7.0 mm; in FIG. 5b the layoutmeasures 13.0 by 5.5 mm; and in FIG. 5c the layout measures 8.2 mm by7.3 mm. In a similar manner, FIGS. 6a-6c show component layouts usingtwo photodetectors 24' and one or two pairs of LED's 22 or 22". In FIG.6a the rectangular layout measures 8.1 mm by 6.8 mm; in FIG. 6b thelayout measures 11.5 mm by 6.8 mm; and in FIG. 6c the layout measures8.6 mm by 7.5 mm.

In FIGS. 4a-6c each LED pair had an area of 3.0 mm by 2.4 mm. Thephotodetectors in FIGS. 6a and 6c had an area of 2.25 mm by 6.25 mm. Inthe remaining layouts the photodetectors each had an area of 4.0 mm by6.25 mm.

FIG. 7 is an electrical schematic of the sensor 20 of FIG. 2. The twoLED's 22a and 22b are connected in parallel and are parallelled byback-biased diodes 23 and 25. The anodes of each pair of components areconnected to respective resistors of values chosen in correspondencewith the drive current to be supplied to the LED's. The connectedcathodes of the LED's are likewise coupled to a biasing resistor. Theresistors may be mounted in line with the flexible printed wiring, suchas the points 32a-32d at which the wiring joins the cable to the pulseoximeter monitor.

FIGS. 8a-8c illustrate a further embodiment of an epidural sensor inwhich the sensor components are located in a hollow bone screw 40. Thescrew 40 is threaded as indicated at 44 to screw into the skull, and thehead of the screw has a slot 42 to turn the screw with an adjustmentinstrument as more clearly shown in the top plan view of FIG. 8b. Aphotodetector 24 and a pair of LED's 22 are located at the bottom of acore of soft, compressible foam material 52 in the center of the screw,as shown in the bottom plan view of FIG. 8c. Above the compressible foam52 the center of the screw is filled with a firm filling 50 of siliconerubber or polyurethane. The electrical leads 26' from the LED's andphotodetector pass through the foam material 52 and the filling 50 andexit through the top of the hollow screw as shown in FIG. 8c.

In use of the sensor embodiment of FIGS. 8a-8c, a hole is drilled in theskull into which the bone screw 40 is screwed. As the bone screw isscrewed into the skull, the oximeter monitor is continuously monitoredfor the onset of oxygen saturation readings. When the bottom of thescrew with the sensor components contacts the dura, oxygen readings willcommence, and will initially occur erratically. As the bone screw isslowly turned the sensor components will make better contact with thedura and the signal quality will improve. The contact between the sensorcomponents and the dura is induced in a gentle manner by thecompressible foam 52, which will readily compress as the components makecontact with the dura to prevent damage to the dura. When consistentreadings occur no further turning of the screw is necessary, as thesensor components are in good surface contact with the dura and willgently ride on the dura due to the compressibility of the foam 52. Thishollow bone screw embodiment is desirable for its ability to completelyblock ambient light from the sensor components and by plugging the burrhole with the bone screw infection of the dura is retarded. The sensorcan be safely left in place in the burr hole for extended periods oftime.

What is claimed is:
 1. A sensor for measuring cerebral oxygen saturation through a burr hole in the skull by optical reflectance comprising:a hollow bone screw; a core of compressive material located in said hollow bone screw and having an upper surface oriented toward the top of said bone screw and a lower surface oriented toward the bottom of said bone screw; and a photodetector and a pair of light emitting diodes located at said lower surface of said core of compressive material, wherein electrical connection is made to said photodetector and said light emitting diodes through said hollow bone screw.
 2. The sensor of claim 1, further comprising a core of relatively less compressive material located within said bone screw above said upper surface of said compressive core.
 3. The sensor of claim 1, wherein a portion of said core of compressive material extends out from the lower end of said hollow bone screw. 